Microstructural engineering and strength-impact toughness prediction in ultra-low carbon bainitic steel

The strength–toughness combination of an ultra-low carbon, low alloyed ‘air-cooled’ steel has been enhanced by developing and refining the granular bainite (GB) structure. Suitable alloy design and thermo-mechanical treatment involving forging, austenitisation (950–1150°C) and air-cooling resulted in the formation of GB matrix along with small islands of martensite–austenite constituents and allotriomorphic ferrite grains. Microstructural characterisation and phase transformation behaviour were studied by scanning electron microscopy and electron backscattered diffraction analysis. The effect of microstructural parameters on the strength-impact toughness combination was investigated. The steel showed an exceptionally low yield ratio, which is beneficial for seismic resistance application.